This invention relates to a fuel cut-off system for an engine coupled to an automatic power transmission with a lockup device.
It is well-known to cut off or interrupt the supply of fuel to an automotive engine when the engine is required to reduce its speed or when the engine is decelerating. The fuel cut-off is to improve fuel economy and reduce undesirable emissions. Specifically, during deceleration at engine speeds above a reference level, the fuel cut-off continues until the engine speed drops to the reference level. When the engine speed reaches the reference level, the fuel supply is recommenced to prevent the engine from stalling.
Interruption and recommencement of fuel supply causes considerable changes in the torque output of the engine. In addition, the torque output of most engines inevitably fluctuates even when the engine operates at a constant speed. These inevitable torque fluctuations are smaller than the torque changes due to interruption and recommencement of fuel supply. In the case of automotive vehicles, such engine torque changes and fluctuations cause vibrations of the vehicle bodies. The higher the engine speed, the weaker or less annoying the vibrations of the vehicle bodies resulting from such engine torque changes and fluctuations.
Some automotive automatic power transmissions including torque converters have lockup devices which act to mechanically couple the crankshaft of the engines to the output shaft of the transmissions. Specifically, when the lockup device assumes its rest position, the crankshaft of the engine and the output shaft of the transmission are coupled via fluid in the torque converter in normal operation. When the lockup device assumes its operative position, the crankshaft of the engine and the output shaft of the transmission are coupled by means of a completely mechanical connection, not via fluid in the torque converter, and thus the torque converter is disabled or locked up. Under these lockup conditions, since the loss of power transmission occurring in the normally-operating torque converter is avoided, fuel economy is improved.
When such a lockup device assumes its rest position, the torque converter has the additional function of absorbing variations in the torque output of the engines and thus smoothing the engine torque output. Such an additional function disappears when the lockup device assumes its operative position.
Usually, the lockup device is designed to assume its operative position when the engine is operating at intermediate and high speeds above a lockup reference value, where the vibrations of the vehicle bodies resulting from the inevitable engine torque fluctuations are relatively weak. The lockup device assumes its rest position when the engine is operating at low speeds below the lockup reference value.
In the case of an automotive engine provided with a fuel cut-off device and coupled to an automatic power transmission with a lockup device, relatively great vibrations may occur during interruption and recommencement of fuel supply if the lockup device is in its operative position and the engine speed is immediately above the lockup reference value.